Evidence of Earliest Oxygen-Breathing Life on Land Discovered

By Wynne Parry | October 25, 2011 01:00pm ET

The first oxygen-dependent life on land may have been bacteria that "eat" pyrite, also known as fool's gold. Similar bacteria still exist at mining waste sites, where pyrite has been discarded creating highly acidic conditions. Above, a modern-day acidic drainage at an abandoned copper mine.

Credit: Courtesy of Kurt Konhauser

A spike in the chromium contained in ancient rock deposits, laid down nearly 2.5 billion years ago, reveals what appears to be the earliest evidence for oxygen-breathing life on land.

The transformation known as the Great Oxidation Event occurred when the atmosphere gained oxygen, an element crucial for nearly all animal life, including humans. The new analysis indicates the earliest estimate to date for the start of the Great Oxidation Event — 2.48 billion years ago. Other research has suggested small amounts of the gas appeared in the oceans and possibly the atmosphere around 2.5 billion years ago.

For this study, the researchers performed more than 2,000 analyses on samples from more than 100 rock formations, including those called banded iron formations, located around the world, from Canada to South Africa.

The rock above is a 2.48 billion-year-old banded iron formation from Australia that contains high concentrations of chromium, which scientists believe is evidence of a pivotal change in the Earth's atmosphere: the arrival of oxygen.

Credit: Courtesy of Stefan Lalonde

Life did exist at the point when chromium levels increased, but it was simple; single cells had yet to come together and begin cooperating as multicellular life forms.
Scientists believe microbes called cyanobacteria living in the ocean kick-started the transformation when they began to photosynthesize. Oxygen, a byproduct of photosynthesis, accumulated in the ocean, then percolated into the atmosphere. Now, oxygen accounts for 21 percent of the air we breathe, and humans need it to survive.

Although the rocks were formed under the oceans, on submerged continental shelves, they accumulated metals, including chromium, which had washed off the continents by rivers and groundwater. The researchers looked at chromium because it is very difficult to dissolve, according to lead researcher Kurt Konhauser, a geomicrobiologist at the University of Alberta.

Before it arrived in what would become these rock deposits, traces of chromium were tied up in other compounds within rocks on land, including pyrite, a shiny gold mineral known as fool's gold. For millions of years, the chromium remained bound up; then about 2.48 billion years ago, something began releasing it into the oceans.[Photos: World's Most Famous Rocks]
That something was a powerful acid, created by a chemical reaction with pyrite, Konhauser said. And in order to get the pH — a measure of acidity — low enough to explain the presence of chromium, sulfuric acid must have been present, he said.

This sulfuric acid must have come from the pyrite at the hands, if you will, of bacteria. These bacteria —  similar species still exist — would have used oxygen taken from the atmosphere to perform an energy-releasing chemical reaction. In essence, the bacteria "eat" the pyrite.
Pyrite contains sulfur, and this reaction forms sulfuric acid. So, Konhauser and colleagues think that the sulfuric acid dissolved the chromium, which made its way to the oceans.

Modern versions of these bacteria are known to live off pyrite discarded by the mining industry, creating highly acidic conditions in water that collects around these waste sites.
Konhauser said he is not aware of any prior work highlighting this milestone in the history of life. "We are the first to explicitly talk about the origin of these organisms on land," he said.
The study was published in the Oct. 20 issue of the journal Nature.

You can follow LiveScience writer Wynne Parry on Twitter @Wynne_Parry. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.

Earth Had Oxygen Much Earlier Than Thought

By Charles Q. Choi, Live Science Contributor | September 25, 2013 01:11pm ET

Oxygen may have filled Earth's atmosphere hundreds of millions of years earlier than previously thought, suggesting that sunlight-dependent life akin to modern plants evolved very early in Earth's history, a new study finds.

The findings, detailed in the Sept. 26 issue of the journal Nature,have implications for extraterrestrial life as well, hinting that oxygen-generating life could arise very early in a planet's history and potentially suggesting even more worlds could be inhabited around the universe than previously thought, the study's authors said.

It was once widely assumed that oxygen levels remained low in the atmosphere for about the first 2 billion years of Earth's 4.5-billion-year history. Scientists thought the first time oxygen suffused the atmosphere for any major length of time was about 2.3 billion years ago in what is called the Great Oxidation Event. This jump in oxygen levels was almost certainly due to cyanobacteria — microbes that, like plants, photosynthesize and exhale oxygen.

However, recent research examining ancient rock deposits had suggested that oxygen may have transiently existed in the atmosphere 2.6 billion to 2.7 billion years ago.

The new study pushes this boundary back even further, suggesting Earth's atmosphere became oxygenated about 3 billion years ago, more than 600 million years before the Great Oxidation Event. In turn, this suggests that something was around on the planet to put that oxygen in the atmosphere at this time.

"The fact oxygen is there requires oxygenic photosynthesis, a very complex metabolic pathway, very early in Earth's history," said researcher Sean Crowe, a biogeochemist at the University of British Columbia in Vancouver. "That tells us it doesn't take long for biology to evolve very complex metabolic capabilities." [7 Theories on the Origin of Life]

Ancient oxygen reactions

Crowe and his colleagues analyzed levels of chromium and other metals in samples from South Africa that could serve as markers of reactions between atmospheric oxygen and minerals in Earth's rocks. They looked at both samples of ancient soil and marine sediments from about the same time period — 3 billion years ago.


The researchers focused on the different levels of chromium isotopeswithin their samples. Isotopes are variants of elements; all isotopes of an element have the same number of protons in their atoms, but each has a different number of neutrons — for instance, each atom of chromium-52 has 28 neutrons, while atoms of chromium-53 have 29.

When atmospheric oxygen reacts with rock — a process known as weathering —heavier chromium isotopes, such as chromium-53, often get washed out to sea by rivers. This means heavier chromium isotopes are often depleted from soils on land and enriched in sediments in the ocean when oxygen is around. These proportions of heavier chromium were just what were seen in the South African samples. Similar results were seen with other metals, such as uranium and iron, that hint at the presence of oxygen in the atmosphere.

"We now have the chemical tools to detect trace atmospheric gases billions of years ago," Crowe told LiveScience.

'Almost certainly biological'

All in all, the researchers suggest atmospheric oxygen levels 3 billion years ago were about 100,000 times higher than what can be explained by regular chemical reactions in Earth's atmosphere. "That suggests the source of this oxygen was almost certainly biological," Crowe said.

"It's exciting that it took a relatively short time for oxygenic photosynthesis to evolve on Earth," Crowe added. "It means that it could happen on other planets on Earth, expanding the number of worlds that could've developed oxygenated atmospheres and complex oxygen-breathing life."
Future research can look for similarly aged rocks from other places, both on and outside Earth, to confirm these findings. "Research could also look at earlier rocks," Crowe said. "Chances are, if there was oxygen 3 billion years ago, there was likely oxygen production some time before as well. How far back does it go?"

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